This project seeks to realise the adventurous goal of "designer magnetism", based around the flexibility and functionality offered by 2D magnetic materials. Of fundamental importance in its own right, but also as a platform to create targeted spintronic functionality, interfacing different van der Waals materials and magnets together promises almost limitless possibilities for tuning magnetic interactions and ordering tendencies and for realising new quantum states and phases. We will realise 2D magnetic materials and heterostructures in thin film geometries. This is crucial not only to unlock their potential for device applications, but will also advance new possibilities for epitaxial engineering that are not achievable in other material systems or using exfoliated 2D materials. Our epitaxial and all-vacuum based synthesis and characterisation approach will open new routes to studying the 2D magnetic materials created using advanced spectroscopic tools. Through this, we will gain much-needed fundamental understanding of how the microscopic magnetic interactions and excitations at play in these layered systems become modified in the ultra-thin limit, where the application of traditional magnetic probes such as neutron scattering becomes impossible or impractical. This, in turn, promises to deliver a step change in our fundamental understanding of low-dimensional magnetism, and through this to pave the way for a host of new spintronic and quantum technologies.

The worldwide demand for digital data storage is increasing exponentially. IBM estimates that 2.5 quintillion of digital data bytes are produced every day on Earth (2.5 x 1018 bytes = 2.5 Exabytes = 2.5 billion Gigabytes). This huge digital data storage demand has two consequences: a) the increased power consumption of data storage servers; b) the perpetual need to develop data storage technologies that meet the increasing demand at reduced cost and power consumption. These issues have prompted the acceleration of research into solid-state memories, which are fast replacing traditional magnetic hard disc drives in almost all consumer electronics and portable devices. In Dec. 2016 and Jan. 2017, the principal investigator (PI) of this travel grant and his co-investigator from Iowa State University, proposed and demonstrated, for the first time, a novel solid-state memory effect in bulk anti-ferroelectric ceramic materials [1-3]. Although this is a very promising discovery, the authors pointed out a few issues that required further investigation, including a relaxation process that significantly limited the signal recovered from a memory cell. In addition, it was acknowledged that the effect was observed in bulk anti-ferroelectric materials, while solid-state memory chips are based on thin films. This modest EPSRC overseas travel grant (< £15k), seeks to swiftly redress these issues by facilitating five weeks travel to three overseas institutions, in two countries, in order to perform specialized experiments on anti-ferroelectric materials and to acquire key skills that will advance our understanding of anti-ferroelectric materials and potentially accelerate the commercialization of this research. By working with leading researchers in the field of data storage technologies at Western Digital (WD) - California, physics of anti-ferroelectric materials at Iowa State University (ISU) and experts in Mossbauer Spectroscopy at the National Institute of Materials Physics (NIMP) in Bucharest, the PI will have the opportunity to study relaxation processes and memory effect in anti-ferroelectric thin films. In addition, the PI will acquire valuable new experimental skills such as: fabrication of anti-ferroelectric materials and their domains imaging using in-situ Transmission Electron Microscopy (ISU), device architecture / solid state memory cell testing (WD) and Mossbauer Spectroscopy (NIMP). This is an excellent value for money low-risk / high-gain EPSRC travel grant, with huge potential for academic, societal and economic impacts. [1]. M. Vopson, X. Tan, 4-state anti-ferroelectric random access memory, Electron Device Letters (2016). [2]. M. Vopson, G. Caruntu, X. Tan, Polarization reversal and memory effect in anti-ferroelectric materials, Scripta Materialia vol. 128, 61-64 (2017). [3]. M. Vopson, X. Tan, Nonequilibrium polarization dynamics in antiferroelectrics, Physical Review B 96 (1), 014104 (2017)

The operation of modern day electronics depends upon electric currents that transport electron charge. However, the electron also possesses intrinsic angular momentum, known as "spin", that is responsible for its magnetic moment. Spin is a quantum-mechanical quantity with two allowed values. We can therefore think of the electron as the smallest possible bar magnet with its north pole pointing either up or down. Ordinarily an electric current transports equal numbers of electrons in the up and down states. However, inside a ferromagnetic material there are more electrons in the up state than the down state; this is the origin of its magnetic behaviour. This means an electric current drawn from a ferromagnet will have a preponderance of up spins. In fact, under certain circumstances in non-magnetic metals, we can arrange for equal numbers of electrons with up and down spins to move in opposite directions so that there is a flow of spin angular momentum without any flow of charge. This is what is meant by a pure spin current. Within a ferromagnet an additional mechanism is available to transport spin current. Rather than the electrons moving, we can think of one electron flipping its spin from up to down and the location of this flipped spin moving from one atom to the next. This mechanism is present even when the material is an electrical insulator and is known as a "spin wave". Ferromagnets are only one of many types of material that have magnetic order. This proposal is concerned primarily with antiferromagnetic materials, where the direction of the spin alternates between up and down for successive layers of atoms. Antiferromagnets have no net magnetic moment, because those on adjacent atoms cancel out, so are generally more difficult to study, and for a long time were thought to be useless in terms of practical applications. However, spin waves also occur in antiferromagnets and so antiferromagnets can be used to transport pure spin current. It was recently observed that the amplitude of a spin current can be enhanced by the insertion of thin antiferromagnetic layers into a stack of ferromagnetic and non-magnetic layers. We have shown that the antiferromagnetic layer is able to transport both dc and ac spin currents, confirming a model that also predicts that spin currents could be amplified by at least a factor of 10 if the thickness of the layer is chosen carefully. This additional angular momentum is drawn from the crystal lattice. Given that a small electric current is usually required to generate a pure spin current, the ability to amplify spin current in the antiferromagnetic layer means that the energy efficiency of devices using spin currents could be significantly improved. One immediate example is a type of magnetic random access memory (MRAM), where spin current is injected into a ferromagnetic layer to reverse its magnetization so as to represent a 0 or 1 in binary code. Reducing power consumption by just a factor of 2 would already make MRAM an attractive alternative to dynamic random access memory (DRAM) within data centre applications. In this project, we will use an ultrafast laser measurement technique to first observe the spin wave modes that exist within antiferromagnetic thin films that may be the order of 10 atomic diameters in thickness. This will be a major achievement since ultrathin films can behave very differently to bulk crystals, and methods for observing their spin waves have yet to be demonstrated. Once we have this information, we will then be able to design multi-layered stacks in which to observe the propagation and amplification of spin currents. Specifically, we will use a time resolved x-ray measurement technique at a synchrotron source that we have already developed and demonstrated. Finally, we will explore how the stacks can be optimised so that they can be used in practical applications such as MRAM.